In
computing an
event is an action that is usually initiated outside the scope of a program and that is handled by a piece of code inside the program. Typically events are handled synchronous with the program flow, that is, the program has one or more dedicated places where events are handled. Typical sources of events include the user (who presses a key on the
keyboard, in other words, through a keystroke). Another source is a hardware device such as a timer. A computer program that changes its behavior in response to events is said to be
event-driven, often with the goal of being
interactive.
Description
Event driven systems are typically used when there is some asynchronous external activity that needs to be handled by a program. For example, a user who presses a button on their mouse. The outside activity causes the event (it
fires), some outside hardware and or software will collect data about the event, and when the program signals that it is ready to accept an event, the event will be
dispatched to the
event handler software that will deal with it.
A program can choose to ignore events, and there may be libraries to dispatch an event to multiple handlers that may be programmed to listen for a particular event. The data associated with an event at a minimum specifies what type of event it is, but may include other information such as when it occurred, who or what caused it to occur, and extra data provided by the event source to the handler about how the event should be processed.
Events are typically used in user interfaces, where actions in the outside world (mouse clicks, window-resizing, keyboard presses, messages from other programs, etc) are handled by the program as a series of events. Programs written for many windowing environments consist predominantly of event handlers.
Events can also be used at instruction set level, where they complement interrupts. Compared to interrupts, events are normally handled synchronously: the program explicitly waits for an event to be serviced (typically by calling an instruction that dispatches the next event), whereas an interrupt can demand service at any time.
Examples
Mouse
With a
pointing device such as a mouse, clicking a
button triggers a "mouse click" event. The programmer would then program the software to respond to this "mouse click" event. Typical mouse events include mouse move and mouse button up/down.
Keyboard
When a user presses a key on a keyboard, the program currently running would receive a keyboard "KeyDown" event along with relevant data such as which key the user pressed.
References
See also
Event handler
Exception handling
Interrupts
Category:Computer programming
